The present invention relates to an encapsulated epoxy adhesive system and, more particularly, to a water based adhesive system employing a hydrophobic curing agent.
Most common epoxy adhesives are prepared from a two-part system comprising an epoxy resin and a curing agent. Prior to application, the two parts are mixed and the resin and curing agent mixture are applied to the bonding site where the resin is cured into a hard adherent mass. The common two-part adhesive system is inconvenient to work with because the two parts must be mixed and the adhesive cannot be applied directly to the bond site. One of the techniques that has been used to make the two part system more convenient has been to microencapsulate the epoxy resin and the curing agent. Encapsulated resin and curing agent do not react. Therefore, a mixture of the capsules can be formed and applied directly to a bonding site. There, when the capsules are broken the resin and curing agent react and the epoxy resin bonds. Encapsulated adhesive systems such as this have been employed with threaded fasteners. The encapsulated resin and curing agent are applied to the fastener. As the fastener is tightened, the capsules break and the adhesive bond is formed.
Some of the problems which have been associated with prior epoxy adhesive systems have been that the curing agents used are relatively hygroscopic and have a short shelf life, and the systems have employed a polymeric vehicle. Capsules of curing agent produced by prior encapsulation techniques have been to some extent moisture permeable and the shelf life of the system has not been appreciably enhanced. In accordance with the present invention these drawbacks have been overcome by an encapsulated adhesive system employing a hydrophobic and therefore less hygroscopic curing agent. In accordance with a preferred embodiment of the invention, the polymeric vehicle is replaced by water without sacrificing the adhesive qualities of a polymer based system.
The system of the present invention has the following advantages:
1. Much higher breakaway torques.
2. Much greater consistency and reproducibility.
3. Much less moisture sensitivity which means coated bolts may not have to be protected from the environment.
4. Much longer pot-life. This enables a 1-part ready-to-apply aqueous based epoxy adhesive.